1. Field of the Invention
The present invention relates to a single-crystal pulling device and a single-crystal pulling method used to manufacture a single crystal and a superconducting magnet applicable to the single-crystal pulling device.
2. Related Art
A semiconductor such as silicon or gallium arsenide consists of a single crystal, and is used for a memory device and the like of a computer having various sizes. It is therefore required for such memory device to be increased in its capacity, be produced at a reduced cost, and improved in its quality.
In order to satisfy these requirements, prior art provides, as one single-crystal pulling method for manufacturing a single crystal, a method in which a magnetic field is applied to a semiconductor material set in a molten state and accommodated in a crucible. In this arrangement, heat convection generated in the melt is suppressed to manufacture a semiconductor having a large diameter and high quality. This method is generally called the Czchoralsk (CZ) method.
An example of a single-crystal pulling device using the conventional CZ method will be described below with reference to FIG. 18. This single-crystal pulling device comprises a pulling furnace 1 which has an opened upper end in an installed (i.e., illustrated) state and in which a crucible 2 is built. Inside the pulling furnace 1, a heater 3 for heating and melting a semiconductor material in the crucible 2 is arranged around the crucible 2, and outside the pulling furnace 1, a superconducting magnet (or magnet device) 30 is arranged. A cylindrical vessel 5 for cooling medium or coolant (i.e., coolant vessel 5), is arranged in the superconducting magnet 30, which includes a pair of superconducting coils 4 (4a and 4b).
In a manufacturing process of a single crystal, a semiconductor material 6 is put in the crucible 2 and heated by the heater 3 so as to melt the semiconductor material 6. A seed crystal, not shown, is downwardly inserted into the crucible 2 through the upper opening, and the seed crystal is then pulled up in the pulling direction 8, vertically as shown, by a pulling machine, not shown, at a predetermined speed. In this manner, crystal is grown in a boundary layer between a solid substance and a liquid substance to thereby generate and form a single crystal. At this time, when the fluid motion of the melt (i.e. molten solution) induced by a heat from the heater 3, i.e., heat convection, is generated, the melt to be pulled up is disturbed, and the yield of single-crystal generation is deteriorated.
Therefore, in order to eliminate such defect or inconvenience, there is provided the superconducting magnet 30 comprising the superconducting coils 4. More specifically, according to the arrangement of such superconducting coils 4, a motion suppressing force is applied to the semiconductor material 6 of the melt by magnetic lines 7 generated by conducting electricity to the superconducting coils 4, and by slowly pulling up the semiconductor material 6 in accordance with the pulling operation of the seed crystal without being convected in the crucible 2. In this manner, so-called a solid-state single crystal 9 is manufactured. Further, in this operation, above the pulling furnace 1, there is arranged a pulling machine for pulling the single crystal 9 along a center line 10 of the crucible 2.
An example of the superconducting magnet 30 used in the single-crystal pulling device of FIG. 18 will be explained hereunder with reference to FIG. 19. The superconducting magnet 30 is constituted by accommodating the superconducting coils 4 (4a and 4b) in a cylindrical vacuum vessel 19 through the cylindrical vessel 5 for coolant, cooling medium or refrigerant, which may be called hereinlater as coolant vessel 5. In the superconducting magnet 30, a pair of superconducting coils 4a and 4b are arranged so as to oppose to each other with reference to a central axial line of the cylindrical vacuum vessel 19 on a horizontal plane. The pair of superconducting coils 4a and 4b are Helmholtz coils which generate a magnetic field along the same transverse direction. As shown in FIG. 18, the magnetic lines 7 which are symmetric with respect to the center line 10 of the cylindrical vacuum vessel 19 are generated and this position of the center line 10 is called as a center of magnetic field.
The superconducting magnet 30 further includes a current lead 11 for guiding a current to the two superconducting coils 4a and 4b, a small helium refrigerator 12 for cooling a first radiation shield 17 and a second radiation shield 18 accommodated in the coolant vessel 5, a gas discharge pipe 13 for discharging a helium gas in the coolant vessel 5, and a service port 14 having a resupply port for resupplying liquid helium. The superconducting magnet 30 has a bore 15, having an upper opening, in which the pulling furnace 1 shown in FIG. 18 is arranged.
FIG. 20 shows a magnetic field distribution of the conventional superconducting magnet 30 of the structure described above. As shown in FIG. 20, in the conventional superconducting magnet 30, a pair of superconducting coils 4a and 4b are arranged so as to oppose to each other. Therefore, the magnetic field gradually increases in size towards the coils in a coil arrangement direction (X-direction), and the magnetic field gradually vertically decreases in size in a direction (Y-direction) perpendicular to the coil arrangement direction. In the conventional configuration described above, because a magnetic field gradient in a bore (opening) range is excessively large as shown in FIG. 20, an effect of suppressing heat convection generated in the melt is unbalanced, and hence, magnetic field efficiency becomes poor.
More specifically, as indicated by a hatched area in FIG. 20, in a region near the center of magnetic field, the magnetic field is not uniform (that is, the magnetic field has a shape of cross which is horizontally and vertically thin and long). For this reason, the suppressing accuracy of heat convection becomes poor, and it is hence difficult to pull up a single crystal having high quality.